The noun reaction in chemistry is a countable/uncountable noun. In what sentences can I use it as an uncountable noun? For example, do the sentences below use it correctly?

  1. Reaction was allowed to proceed to synthesize substance X.
  2. Substance X was used in reaction.
  3. The temperature was adjusted at 100F so as to carry out reaction.

And, in what sentences can I use it as a countable noun?

English is my second language, and my first language doesn't have articles.

2 Answers 2


Combining substance A either with B or with C, we get two completely different reactions.

"Reaction" is uncountable in the sense that a little bit of a reaction and a huge amount of a reaction is still just one reaction: Combine a milligram of magnesium with water and you get a reaction. Combine a ton of magnesium with a cubic meter of water and you get still just one reaction (although one that will get noticed in a huge area).

"Reactions" are countable when you have different reactions. Like combining different substances will lead to different reactions.

  • Thank you! Your explanation "a little bit of a reaction and a huge amount of ..." makes it clear a lot. But I still have a hard time with the countable usage. Can I say "reaction" is countable when it has a modifier(s)? Like these below. - a two-step enzymatic reaction/ - an ammonia synthesis reaction/ - a hybridization reaction/
    – Kay
    Jun 23, 2014 at 12:14
  • @Kay "A little bit of a reaction." You're using it as countable: note the "a". "A two-step enzymatic reaction." countable again: "a".
    – Jay
    Jun 23, 2014 at 15:42
  • Thank you. "A little bit of a reaction." is written by gnasher729 in the answer, not by me... I'm not still getting it, though...
    – Kay
    Jun 24, 2014 at 0:28

I don't want to be dogmatic, but I'm hard pressed to think of an example where you can use "reaction", in the sense of a chemical reaction, as an uncountable noun.

When you mix two chemicals, you get "a reaction". There is one reaction -- it is countable. If we were discussing mixing different chemicals, we would be discussing multiple reactions. "What REACTIONS did you get when you mixed sulfur with each of the four unknowns?"

In all of your examples above, you should say either "a reaction" or "the reaction", depending on the larger context.

Can anyone on here give an example where "reaction" is used as an uncountable noun? I did a brief Bing search and couldn't find any.

  • 1
    It could be used in other contexts, I think, but perhaps not one about chemical reactions. For example, I found this "Action and reaction are always equal, but in opposite directions," on Google Books. Jun 23, 2014 at 16:18
  • Thank you for the discussion. So, "reaction" in chemistry sounds like countable in most cases. Here are the reasons why I'm confused. (1) Its countability in chemistry varies in different dictionaries. For example, it's U/C in Longman: ldoceonline.com/dictionary/reaction, while it's C in Cobuild. (2) In papers and patents in the field, I see its uncountable usage just like the three examples in my question (1. Reaction was allowed to proceed...). Just for making the sentence shorter and simpler...?
    – Kay
    Jun 24, 2014 at 0:24
  • @kay When people are writing titles or bullet lists, they often omit articles and pronouns and otherwise shorten the sentence. A bullet point is not necessarily a grammatically correct sentence. Like someone might give a bullet point of "1. Started reaction", when the full sentence would be, "The researchers started the reaction", etc.
    – Jay
    Jun 24, 2014 at 13:27
  • Exactly. I write papers and patents, and making titles and bullet lists short and precise is very important. But my examples are from the body of document, not from titles or lists. I'd appreciate if anyone could help me use "reaction" correctly. Thank you!
    – Kay
    Jun 25, 2014 at 0:45
  • @kay Outside of a title, bullet list, or other such abbreviated statement, we would normally write, "The reaction was allowed to proceed", NOT "Reaction was allowed to proceed". From the "1." you had in front of it in your example, I was assuming it was a title or bullet item. If you're quoting someone else ... well, without more context, I'd say they probably just used the word incorrectly.
    – Jay
    Jun 25, 2014 at 13:20

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